This invention relates to gas turbine engines, and more particularly to an annular flow path in the compression section of such an engine.
A gas turbine engine has a compression section, a combustion section and a turbine section. An annular flow path for working medium gases extends through the engine. An inner wall and an outer wall bound the annular flow path. In typical prior art constructions, arrays of stator vanes extend radially inwardly from the outer wall and rows of rotor blades extend radially outwardly from the inner wall. The arrays of stator vanes and the arrays of rotor blades are interdigitated. In the compression section, the walls of the flow path gradually converge with respect to each other. One such construction having a flow path converging at both the outer wall and the inner wall is illustrated in U.S. Pat. No. 2,869,820 to Marchant et al. entitled "Rotors For Axial Flow Compressors Or Turbines." Another construction having a converging outer wall, conical in shape, and a cylindrical inner wall is shown in U.S. Pat. No. 2,672,279 to Willgoos, entitled "End Bell Construction." U.S. Pat. No. 2,801,071 to Thorpe, entitled "Bladed Rotor Construction" is a construction having a conical inner wall and a cylindrical outer wall.
In each of these constructions the rotor assembly and stator assembly cooperate to compress the working medium gases. As the gases are compressed the temperature and the total pressure of the gas rises. Across each array of rotor blades the increase in total pressure is accompanied by an increase in static pressure.
It is common practice to express static pressure distribution on an airfoil and across the airfoil in terms of a pressure coefficient P. The pressure coefficient P is defined as the dimensionless ratio of the static pressure rise between an upstream point and a point on the airfoil to the dynamic or velocity pressure at the upstream point. This may be represented by the formula ##EQU1## where p represents the pressure at any point on the airfoil,
P.sub.o represents the pressure at a distance upstream from the airfoil, and PA1 1/2 .rho.V.sup.2 is the upstream velocity or dynamic pressure.
The aerodynamic loading across an airfoil is defined as the static pressure rise across the entire airfoil divided by the inlet dynamic pressure or velocity pressure. During operation, high aerodynamic loadings on airfoils are often accompanied by separating flow. Because the airflow is in the direction of increasing static pressure in a compressor, there is a tendency of the flow to "separate" from the blade and wall surfaces.
Separation decreases the efficiency of the array of rotor blades and in extreme cases can result in a phenomenon known as surge. Compressor surge is generally characterized by a complete stoppage of flow, or a flow reversal, through the compressor system, or by a sharp reduction of the airflow handling ability of the engine for particular operating rotational speed. The latter is called a "hung surge." The engine will generally not respond to throttle increases properly when such a condition exists.
Accordingly, scientists and engineers are seeking to improve the surge margin and efficiency of an array of rotor blades by affecting the distribution of aerodynamic loading across the airfoils.